Hi,
This question has not been asked and I thought this
could be usefull for many beginners like me: how to add a new class locally? Without any functionality I tried to add to the same folder where my solver is located just the frames for a new class. First, I included in the Make/files the new .C file. Then, naming the class sV the files
are:

******************
And in the main solver .C-file
I include the sV.H before main
and try to create an object inside
the main by stating

sV sob();

I tried also Foam::sV sob();
I also removed the namespaces from .C and .H
and tried all kinds of combinations but these did not work (to mention: removing the namespaces
produced a compiled version but no functionality
could be added to the class..). I think I should add some header
files (such as standard libraries in c) to my new class but which headers are those?

What about the namespace: why is practically everything (?) declared inside a namespace Foam
and what consequences does this have?
If someone could clarify and I can summarize
my experiences thereafter!

your example looks alright, post the code and error message next time, otherwise its hard to say anything.

Namespace:
Namespaces are used to get some order in big projects and to avoid name conflicts. A class name like "vector" is relatively likely to used by many programmers. So to make sure to get the Foam "vector" you write "Foam::vector". Inside Namespaces you can have more Namespaces. A Foam example would be "Foam::fvc::div" and "Foam::fvm::div", which are explicit and implicit implementations of the divergence of a field.

Thank you for the answer!
To repeat, in the folder /mysolver/ I wanted
to add a new class sV. Thus I add here the
files sV.C and sV.H and add the sV.C in the
/Make/files. The #include "sV.H" is added before
main in the mysolver.C. I would just like to
first make a function returning an integer.
Here are the files: